One guy has problem on making an pretty unstardard thread. I can't help on that one: TR 15 20 TPI.
ACME and trapetzoidal threads are a litle different and well documented. No problem there. Bastard threads are not that incommon.
Problem description: small internal thread is pretty had to make in lathe single point threading.
1: How small trapetzoidal/acme thread is practical to do single pointing? Looks like threading inserts for under 16 mm shanks are not that common.
https://www.shop-apt.co.uk/threading-inserts.html2: Making unstardard taps seems to be possible, but trapezoidal thread seems to be pretty hard form cutting. Long taper or series tap? Other ideas?
Pekka